The present invention relates generally to implantable cardiac stimulation devices, and in particular to multisite stimulation devices in which multiple sensing vectors are provided for accurate detection of cardiac events, their origin, and their conduction time and direction, by incorporating one or more passive sensing electrodes in each chamber of the heart.
In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart at a desired energy and rate. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
Cardiac pacemakers conventionally stimulate a heart chamber by applying current pulses to cardiac tissues via two electrodes, a cathode and an anode. Standard pacing leads are available in either of two configurations, unipolar leads or bipolar leads, depending on the arrangement of the electrodes of a particular lead. A unipolar pacing lead contains a single electrode, normally the cathode, which extends pervenously distal from the pacemaker in an insulating enclosure until it is adjacent to the tip of the lead where the insulation is terminated to provide for electrical contact of the cathode with the heart tissue. The anode provides a return path for the pacing electrical circuit. For a unipolar lead, the anode is usually the pacemaker case.
A bipolar lead contains two electrodes within an insulating sheath, an anode that extends distal from the pacemaker to a position adjacent to, but spaced from, the electrode tip, and a cathode that also extends distal from the pacemaker, but terminates a short distance distal of the anode, at the lead tip. The anode commonly takes the form of a ring having greater surface area than the cathode tip. An insulating barrier separates the cathode and anode of a bipolar lead. In present-day pacemakers, circuits for pacing and sensing that determine tip, ring and case electrode connections are provided. Thus, the pacemakers can be programmed via telemetry for either bipolar or unipolar operation with respect to either sensing or pacing operations.
A single-chamber pacemaker delivers pacing pulses to one chamber of the heart, either one atrium or one ventricle, via either a unipolar or bipolar electrode. Single-chamber pacemakers can operate in either a triggered mode or a demand mode. In a triggered mode, a stimulation pulse is delivered to the desired heart chamber at the end of a defined time-out interval to cause depolarization of the heart tissue (myocardium) and its contraction. The stimulating pulse must be of sufficient energy to cause depolarization of the heart chamber, a condition known as xe2x80x9ccapture.xe2x80x9d The lowest stimulation output required to achieve capture is termed xe2x80x9cthreshold.xe2x80x9d The pacemaker also delivers a stimulation pulse in response to a sensed event arising from that chamber when operating in a triggered mode.
When operating in a demand mode, sensing and detection circuitry allow for the pacemaker to detect if an intrinsic cardiac depolarization, either an R-wave or a P-wave, has occurred within the defined time-out interval. If an intrinsic depolarization is not detected, a pacing pulse is delivered at the end of the time-out interval. However, if an intrinsic depolarization is detected, the pacing pulse output is inhibited to allow the natural heart rhythm to preside. The difference between a triggered and demand mode of operation is the response of the pacemaker to a detected native event.
Dual chamber pacemakers are now commonly available and can provide either trigger or demand type pacing in both an atrial chamber and a ventricular chamber, typically the right atrium and the right ventricle. Both unipolar or bipolar dual chamber pacemakers exist in which a unipolar or bipolar lead extends from an atrial channel of the dual chamber device to the desired atrium (e.g. the right atrium), and a separate unipolar or bipolar lead extends from a ventricular channel to the corresponding ventricle (e.g. the right ventricle). In dual chamber, demand-type pacemakers, commonly referred to as DDD pacemakers, each atrial and ventricular channel includes a sense amplifier to detect cardiac activity in the respective chamber and an output circuit for delivering stimulation pulses to the respective chamber.
If an intrinsic atrial depolarization signal (a P-wave) is not detected by the atrial channel, a stimulating pulse will be delivered to depolarize the atrium and cause contraction. Following either a detected P-wave or an atrial pacing pulse, the ventricular channel attempts to detect a depolarization signal in the ventricle, known as an R-wave. If no R-wave is detected within a defined atrial-ventricular interval (AV interval or AV delay), a stimulation pulse is delivered to the ventricle to cause ventricular contraction. In this way, rhythmic dual chamber pacing is achieved by coordinating the delivery of ventricular output in response to a sensed or paced atrial event.
Mounting clinical evidence supports the evolution of more complex cardiac stimulating devices capable of stimulating three or even all four heart chambers to stabilize arrhythmias or to re-synchronize heart chamber contractions. Reference is made to Cazeau et al., xe2x80x9cFour Chamber Pacing in Dilated Cardiomyopathy,xe2x80x9d Pacing Clin. Electrophsyiol., 1994, 17 (11 Pt 2):1974-9. In order to achieve multi-chamber or multi-site stimulation in a clinical setting, conventional dual-chamber pacemakers have been used in conjunction with adapters that couple together two leads going to different pacing sites or heart chambers. Reference is made to U.S. Pat. No. 5,514,161 to Limousin.
In certain currently available devices for multi-chamber pacing, adapters are no longer required. The connection between leads is hardwired internally in the stimulation device connector block, coupling the ventricular leads to the ventricular channel and the atrial leads to the atrial channel. While this design advantageously eliminates the need for adapters, the hardwire connections preclude the ability to distinguish between cardiac signals arising from a right chamber of the heart from cardiac signals arising from a left chamber of the heart. This obligatory coupling of the right and left heart chambers also prevents introducing separate timing between stimulation pulses delivered to the right and left chambers. Responding with a programmable delay between a sensed event in one chamber and delivery of an output pulse to the other chamber is thus made impossible.
Since one goal of multi-chamber stimulation is to provide a re-synchronization of the heart chambers in order to improve heart function in patients suffering from congestive heart failure, accurate detection of cardiac signals as well as recognizing the origin of a depolarization are important in achieving a desired therapeutic benefit from stimulation therapy. For example, it may be desirable to detect an intrinsic depolarization occurring in the right ventricle so that a stimulation pulse may be delivered to the left ventricle at a predefined time relative to the detected right ventricular depolarization in order to achieve optimal ventricular synchrony.
Traditionally, intrinsic cardiac events are detected by sensing the internal electrocardiogram (EGM) signals through conventional bipolar or unipolar sensing electrodes. Numerous problems can exist, however, in performing accurate cardiac event sensing. Various signals may be erroneously detected as a P-wave or an R-wave. Such signals include far-field signals (i.e., signals arising from the depolarization of another heart chamber), cross talk (i.e., signals arising from stimulation delivered in another heart chamber), premature atrial or ventricular depolarizations, depolarization of nearby skeletal muscle, or noise.
Erroneous detection or interpretation of the native cardiac rhythm will lead to inappropriate delivery or withholding of stimulation. Either of these results could have adverse effects on the patient""s condition or even be life threatening. When sensing in three or all four chambers of the heart, the importance of accurate sensing and the possibility of erroneous sensing increase compared to less complicated dual chamber applications.
Relying on traditional unipolar or bipolar sensing in just the right atrium and the right ventricle, therefore, may not be adequate in multi-chamber stimulation devices. Discrimination between depolarization signals arising from the chamber being sensed and cross talk, noise or far-field signals becomes more complex when sensing and stimulating in three or even all four chambers of the heart.
During clinical assessment of patients, physicians frequently use 12-lead external ECG studies to monitor timing features of intrinsic cardiac signals in order to diagnose and monitor the progression of congestive heart failure. For example, the duration of the QRS signal shown an ECG is known to increase with worsening heart failure. A 12-lead ECG study can provide a physician with detailed information upon which the physician can base treatment decisions. However, in order to perform a 12-lead ECG study, the physician must be present and cannot practically monitor changes in the ECG signals continuously for the purposes of modulating treatment in response to improving or worsening heart condition. Therefore, it would be advantageous to obtain the kind of detailed information available from a 12-lead ECG study from internal EGM signals received by an implanted device. The device may then respond immediately in administering stimulation therapy such that the patient is receiving optimal therapy at all times.
There remains an unmet need for a multi-chamber cardiac stimulation device that allows accurate, reliable sensing of cardiac events and immediate stimulation therapy response. To this end, clear identification of the origin of a sensed event is needed as well as detailed timing information relating to specific cardiac events and diagnostic measures of changing heart condition. Therefore, a multi-chamber cardiac stimulation device capable of detailed sensing of events in all cardiac chambers is desirable.
The present invention addresses this need by providing a multi-chamber implantable cardiac stimulation device with associated leads and a method for accurate sensing of cardiac signals from all heart chambers including detection of the originating location of a cardiac signal. One or more passive sensing electrodes are provided on a lead placed in each heart chamber in addition to any desired stimulating electrodes. The passive sensing electrodes may be used for sensing a cardiac event within a heart chamber and are further used to create a set of sensing vectors for receiving multiple electromyogram signals from across cardiac chambers. Through processing of inter-chamber sensing vectors, for example subtracting or adding two or more inter-chamber signals, the origin of the detected events may be determined. Detailed information, such as QRS duration or P-wave duration as well as intervals between events, may also be gathered for monitoring changes in conduction or heart failure condition allowing the stimulation device to immediately administer stimulation therapy as necessary.
The foregoing and other features of the present invention are realized by providing an implantable cardiac stimulation device equipped with cardiac data acquisition capabilities. A preferred embodiment of the stimulation device includes a control system for controlling the operation of the device; a set of leads for delivering atrial and ventricular stimulation pulses and receiving cardiac signals through combinations of passive sensing electrodes that provide both intra-chamber and inter-chamber signals; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; pulse generators for generating atrial and ventricular stimulation pulses; and a switching circuit for connecting desired electrode combinations to sensing circuits for sensing a set of EGM signals or to a pulse generator for stimulating in a desired heart chamber. In addition, the device includes memory for storing operational parameters for the control system. The device also includes a telemetry circuit for communicating with an external programmer.
In a preferred embodiment, a lead possessing one or more, preferably two, passive sensing electrodes in addition to electrodes used for pacing stimulation or shocking is placed in each heart chamber. Switching circuitry allows connection of multiple intra-chamber and inter-chamber combinations of the passive sensing electrodes to be connected to the sensing circuits. Digital processing of these signals, such as adding, subtracting, or other manipulation of digitized signals, is performed to remove undesired noise, far-field signals, or cross-talk. Further processing allows analysis of wavefront propagation through or across heart chambers, detailed analysis of signal durations and intervals between detected events, and determination of a signal origin. This information is used by the device control system for determining when and where stimulation therapy should be delivered.
The system and method of the present invention thus provide accurate sensing of cardiac signals in a multi-chamber stimulation device allowing for precise analysis of the timing and location of cardiac events and changes in cardiac function. This analysis improves device performance by allowing stimulation therapy to be delivered according to an accurate assessment of cardiac activity and function.